1. Field of the Invention
The present invention relates to a centrifugal separator with continuous discharge, specifically to a centrifugal separator wherein a slurry is continuously separated into a sludge and a liquid, and the separated sludge and liquid are continuously discharged.
2. Description of the Related Art
Conventionally, a centrifugal separator, such as the countercurrent or counterflow type of centrifugal separator shown in FIG. 1, comprises a rotating bowl 1, a screw shaft 2 positioned concentrically within the interior of the bowl 1, and a screw 3 mounted, for example, by being welded, in a spiral manner on the outer circumference of the screw shaft 2 so that flights 3a of the screw 3 are spaced from each other by a pitch 7.
The rotating bowl 1 and the screw 3 rotate in the same direction. However, the rotating bowl 1 and the screw 3 rotate at different speeds. For this reason, a slurry fed into the rotating bowl 1 is separated into a sludge and a liquid as a result of the centrifugal force in the rotating bowl 1, and the sludge is precipitated onto a inner peripheral surface 1a of the rotating bowl 1, transferred to a left side section 1b by the screw 3 as shown in FIG. 1, during which the sludge is concentrated. Then the sludge is discharged to the outside from a sludge discharge port 5 at one end of the rotating bowl 1, while the separated liquid flows through a flow channel between the screw flights and overflows from a liquid discharge port 4 at the other end of the rotating bowl 1, that is, on the right side of FIG. 1, and is discharged to the exterior of the centrifugal separator.
In addition to the countercurrent flow system shown in FIG. 1, other types of centrifugal separators variously contrived are used, such as the concurrent type. One feature these types of centrifuges have in common is the fact that they are constructed to provide a rotating bowl and a screw for scraping the sludge from the rotating bowl.
The problems to be solved by the present invention will be explained in relation to the helical angle of the screw and the width of the flow channel in centrifugal separators constructed to have such a screw.
In the centrifugal separator shown in FIG. 2, the helical angle .theta. of the screw 3 is larger than for the unit shown in FIG. 1. When the helical angle .theta. of the screw 3 is made larger as in this centrifugal separator, the width of the flow channel of the screw increases and the length of the flow channel is decreased. Conversely, when the helical angle .theta. is made smaller as in FIG. 1, the width of the flow channel of the screw decreases and the length of the flow channel is increased.
When the flow channel is long, the flow rate of the slurry is generally speeded up, so that turbulent flow is produced, resulting in that the efficiency of the sludge precipitation is reduced and, as a result, the cleanliness of the separated liquid deteriorates.
Accordingly, in order to avoid the situation as mentioned above, the centrifugal separator is usually constructed with a large helical angle and a wide but short flow channel. However, when the width of the flow channel is increased, the flow rate is reduced, but swirling flows are produced between the screw flights in the cross sectional direction at right angles to the flow channel, resulting in the problem of poor separation of the sludge. For this reason, to improve the effective separation, there are double screw type centrifuges in which the flow channel is shortened and its width is decreased.
In the double screw type, two flow channels are formed in the rotating bowl, which is opposed to the centrifugal separators having a single flow channel as in FIG. 1 and FIG. 2 and therefore referred to as the single screw type. Specifically, in the single screw type, one flow channel is formed with one screw extending from a slurry inlet orifice 6 to the sludge discharge port 5 or the liquid discharge port 4. In contrast, in the double screw type, a plurality of slurry inlet orifices are divided into an equal number and two flow channels are formed with two screws, extending as far as each of the previously mentioned discharge ports.
In the case of the double screw type, it is theoretically possible to reduce the amount of turbulent flow in the flow channel and the amount of swirling flows in the cross sectional direction. However, in actual fact, the balancing of the feed of the slurry between the two flow channels cannot be performed very well. For example, because of plugging up of the slurry inlet orifice for one of the flow channels, a difference in the amount of sludge produced or conveyed, and other effects which may be encountered, it is difficult to uniformly distribute the slurry in the double screw type. For this reason, it frequently happens that the properties of the sludges and liquids separated from each other inside the two flow channels are not the same.
Since the efficiency of the centrifugal separator is represented by the flow channel having worse properties of the separated sludge or the separated liquid, as previously discussed, the performance of the centrifugal separator is often deemed to be not satisfactory and the centrifugal separation is often deemed to be not stable, so the centrifugal separator is inadequate unless the properties of the sludges and the liquids for the two flow channels are the same.
As outlined above, the difference in screw velocity is made small, and/or the helical angle of the screw is made large, so that turbulent flow lessens, whereby the precipitation effect is being increased and the unit transport capacity of the sludge becomes large. Thus, various means are employed to increase the solids recovery rate and improve the quality of the overflow liquid. However, even when the width of the flow channel is increased and the rate of flow in the direction of flow is reduced, in actual fact, it frequently happens that an improvement in separation performance is not obtained.
This is because other conditions as well as a problem with the Reynolds Number affect the flow state considered in a static system. For example, inside the rotating bowl to which centrifugal force is applied, the motion of the liquid in the flow channel is three dimensional and very complicated. For this reason, the effect of the application of centrifugal force is diminished and turbulent flow is easily produced.
Whichever model of centrifugal separator is used, if the balance between the separated sludge and the separated liquid is not satisfactorily adjusted, the performance is not good. Within the centrifugal separator, in the case of a centrifugal extractor, even if the balance between the sludge and the liquid is not satisfactorily adjusted, the recovery rate of the solid material can be compensated by the addition of a coagulant. However, in the case of a centrifugal concentrator which does not utilize a coagulant, a wide variety of devices must be used in order to discharge a clean liquid after separation, in other words, in order to increase the solid recovery ratio.